docs: def __undo_filter_sub(self, scanline):
          """Undo sub filter."""
          ai = 0
          # Loops starts at index fu.
          for i in range(self.fu, len(scanline)):
              x = scanline[i]
              a = scanline[ai]  # result
              scanline[i] = (x + a) & 0xff  # result
              ai += 1